The present invention relates to a meltblown fibrous insulation media of thermoplastic fibers and, in particular, to a meltblown fibrous insulation media of thermoplastic fibers which is especially suited for use as an acoustical insulation and the method of making such an insulation.
Fibrous insulation media are used for many thermal and/or acoustical applications including but not limited to the acoustical insulations in appliances which reduce the sound emitted into the surrounding areas of a home, acoustical insulations used in office partitions and wall panels, and the acoustical insulations used in vehicles and aircraft which function to isolate the passenger compartment from unwanted sounds or sound levels occurring outside of the passenger compartment. Currently, there are several forms of fibrous acoustical insulation media used as acoustical insulations for such applications and especially, for vehicle passenger compartments. One form of acoustical insulation used in vehicles is cotton shoddy. While this form of acoustical insulation media is inexpensive, it does not perform particularly well when compared to other automotive insulations currently on the market. Other forms of acoustical insulation media used in vehicles include a fibrous mat with a separate cover layer (a scrim, non-woven fabric, film or foil) laminated to a major surface of the mat such as marketed by Minnesota Mining and Manufacturing Company under the trademark xe2x80x9cTHINSULATExe2x80x9d (also see U.S. Pat. No. 5,298,694) and such as manufactured and sold by Johns Manville International, Inc.
The process for producing the acoustical insulation media manufactured and sold by Johns Manville International, Inc., essentially includes three processes. In the first process, a thin but meltblown tightly bonded cover stock is formed having a basis weight of about 0.75 oz/yd2 or another cover stock, such as but not limited to a spun bond cover stock is formed. In the second process an air-laid, non-woven mat or fibrous layer of loose lofty randomly oriented meltblown thermoplastic fibers, e.g. fibers having a mean diameter of about 13.5 microns, and of the required thickness is formed. In a third process a heated pin or calendar roll collates a layer of cover stock onto each major surface of the mat or fibrous layer and, through the heated pins of a pin or calendar roll, heat point bonds the layers of cover stock to the major surfaces of the mat. As discussed above, the resulting product is a fibrous acoustical insulation media with a fibrous core layer of loose lofty fibers encapsulated between two surface layers of cover stock that are heat point bonded to the fibrous core layer. The loose fibers within the media provide an effective surface area for good acoustical absorption of sound waves and the films provide airflow resistance barriers for additional acoustical absorption of sound waves. The heat point bonding of the layers of cover stock to the fibrous core layer provides the acoustical insulation media with added integrity and improves the xe2x80x9chandle-abilityxe2x80x9d of the product. While fibrous acoustical insulation media, such as this media, provide acceptable sound absorption for many applications, there has remained a need for acoustical sound absorption media with equal or better sound absorption properties, that can be more economically produced.
The fibrous insulation media of the present invention and the method of making the fibrous insulation media of the present invention provide acoustical insulation media with equal or better sound absorption properties than the acoustical insulation media of Johns Manville International Inc. discussed above and media which can be more economically produced than the acoustical insulation media of Johns Manville International Inc. discussed above. While, the fibrous insulation media of the present invention is especially suited for use as an acoustical insulation, the fibrous insulation media also may be used for applications other than acoustical applications. When the fibrous insulation media of the present invention is used as an acoustical insulation media, the fibrous insulation media is formed from a non-woven mat of thermoplastic fibers having a mean diameter of less than about 13 microns. For acoustical applications, the media has a density of less than about 60 Kg/m3; a Fraiser air permeability of less than 75 cubic feet per minute per square foot of media surface area; and first and second major surfaces and a fibrous core with at least one of the major surfaces having a thin integral skin thereon. The skin is formed by melting fibers at and immediately adjacent the major surface of the non-woven mat to form a thermoplastic melt layer which is subsequently solidified into an air permeable skin on the major surface of the mat. The thermoplastic fibers of the mat are point bonded together at spaced apart locations to increase the integrity of the mat and preferably, increase the thickness of the mat adjacent the point bonded locations.
Preferably, the method of forming the fibrous insulation media of the present invention is an on-line process which includes: air laying thermoplastic fibers having a mean fiber diameter of less than about 15 microns (less than 13 microns for acoustical media) to form a non-woven mat; melting the thermoplastic fibers at and immediately adjacent at least one of the major surfaces of the mat to form a thermoplastic melt layer on the major surface(s) of the mat; subsequently cooling the thermoplastic melt layer(s) to form a thin, integral thermoplastic skin on the major surface(s) of the mat; and point bonding the thermoplastic fibers of the mat together at spaced apart locations to increase the integrity of the mat and preferably, increase the loft of the mat adjacent the point bonds by displacement of some of the thermoplastic fibers from the locations of the point bonds.
The thermoplastic fibers at and immediately adjacent one or both of the major surfaces of the mat can be melted to form a thermoplastic melt layer on the major surface or surfaces of the mat by flame treating, infrared treating or corona treating the surface or surfaces of the mat. However, preferably, the thin, integral skin is formed on one major surface of the mat by passing the mat between a heated nip or calendar roll with a smooth surface and a backing roll or integral skins are formed on both major surfaces of the mat by passing the mat between two heated nip or calendar rolls with smooth surfaces. Preferably, the major surface of the mat on which a skin is being formed is pressed against the heated surface of a nip or calendar roll by compressing the mat between the heated nip or calendar roll and a match or backup roll or by compressing the mat between two heated nip or calendar rolls. It is believed that the compression of the mat brings more fibers into contact with the heated surface of the nip or calendar roll and increases the density of the mat at and adjacent the heated surface of the nip or calendar roll for better heat transfer from the nip or calendar roll into the thermoplastic fibers of the mat. The result is a better melting of the thermoplastic fibers at and immediately adjacent the major surface of the mat in contact with the heated surface of the nip or calendar roll to form a melt layer on the major surface of the mat that is subsequently cooled and solidified to form an air permeable skin. When a skin was formed on a major surface of a mat without compressing the mat between a heated nip or calendar roll and a match or backup roll or another heated nip or calendar roll, the quality of the skin formed, for acoustical applications, was considerably inferior to the skin formed by compressing the mat between a heated nip or calendar roll and a match or backup roll or another heated nip or calendar roll.
The compression of the mat between a heated nip or calendar roll and a match or backup roll or another heated nip or calendar roll, decreases the thickness of the mat. Accordingly, the thickness and resiliency of the non-woven mat being introduced into the skin forming station of the process line must be sufficient to accommodate the decrease in thickness caused by the skin forming operation without permanently decreasing the thickness and the sound absorption properties of the mat below acceptable levels. In a preferred embodiment of the invention, the mat is made more resilient by forming the mat with thermoplastic fibers formed from a polymeric material with a nucleating agent.
Preferably, the point bonds are formed using the heat generated solely from the pressure exerted on the fibers by the pins of an unheated pin or calendar roll assembly. While the point bonds can be formed using heated pins of a heated pin or calendar roll assembly, the heat from the heated pins of such an assembly causes the thermoplastic fibers contacted and adjacent the heated pins to shrink down to form a point bond. When using unheated pins to form the point bonds, at least some of the thermoplastic fibers present along the paths of pins through the mat are pushed away or displaced from the paths of the pins thickening the mat adjacent the point bonds and leaving only a thin layer of thermoplastic fibers to form the point bonds through the heat generated by the pressure applied by the pins to the remaining thin layer of thermoplastic fibers. Thus, rather than decreasing the thickness of the mat which would decrease the sound absorption properties of the mat, the use of unheated pins maintains or in effect increases the thickness of the mat while increasing the integrity of the mat through the point bonding of thermoplastic fibers within the mat.